In the case of high-power devices, the problems of electric contacts on the active parts of the unit where high currents pass and where there are problems of heat dissipation lead to the use of pressed contacts. These form a contact on a large surface, allow heat dissipation from both surfaces of the wafer and appreciably reduce the thermal impedance.
However, for all new devices of such types as rapid thyristors, power transistors, power integrated circuits and circuits comprising several functions in the same encapsulation, there are, on a single surface, various zones which must be coated with metal layers while remaining insulated from one another. If, further, as is the case for the units cited hereinabove, these zones overlap greatly, the formation of a contact by pressure on some of these zones only sets a difficult problem since it is essential to eliminate the dangers of short circuits.
One known solution consists in cutting in the surface having overlapping zones, compartments including the zones which must be insulated from the metal contact when the latter is pressed against this surface, during the formation of the semiconductor structure from a monocrystalline wafer.
This solution has the disadvantage of complicating the process for forming the semiconductor structure and correlatively the electrical performance of the structure.
Another known solution consists in forming on these various zones metal layers having different thickness. A metal contact with a very plane surface pressed against the surface bearing these metal layers then comes into contact only with the thickest layers. Two metal layers with different thicknesses are generally formed in two successive stages. During each of these stages, firstly metal is deposited by evaporation in a vacuum over the whole surface of the plate, then the metal deposited on some zones is removed by photoengraving. Each of these stages is expensive. Further, the differences in thickness obtained are small (15 to 20 microns) and make it necessary to take precautions to prevent short-circuits. The use of a single metal deposit by evaporation in a vacuum can be considered. But the following photoengraving operation must then allow the metal to be removed completely in some zones, to keep its whole thickness in other zones and to reduce its thickness without removing it completely in still other zones. Known photoengraving methods do not make it possible to cary out such an operation economically and reliably.
Preferred embodiments of the present invention provide a method of manufacturing power semiconductors with pressed contacts in which various zones of a surface of a semiconductor wafer can be formed economically with layers which are in good ohmic contact with the wafer and which have large differences in thickness from one zone to another together with a perfectly plane upper surface.